1. Field of the Invention
The present invention relates to a detector for detecting an impact caused by a collision of an automotive vehicle. The collision impact is represented by a vehicle deceleration exceeding a predetermined limit.
2. Description of Related Art
An example of a collision detector is shown in JP-A-2003-16892. The essential structure of the collision detector is shown in FIG. 11 and various friction torques associated with rotation of its rotor are shown in FIGS. 12A and 12B attached hereto. As shown in FIG. 11, a rotor 110 is rotatably supported on a shaft 100 which is in turn fixed to a housing. The rotor 110 has a cylindrical portion 120 for supporting a coil spring 130 (better seen in FIGS. 12A and 12B). The coil spring 130 biases the rotor 110 in one direction around the shaft 100, and the rotor 110 rotates in the other direction (clockwise direction in FIG. 11) against the biasing force of the coil spring 130 when a deceleration exceeding a certain level occurs. A pair of contacts, a stationary contact 150 and a movable contact 140, are closed by the rotation of the rotor 110, thereby detecting a vehicle collision.
The shaft 100 and the coil spring 130 do not rotate when the rotor 110 rotates. Therefore, as shown in FIGS. 12A and 12B, friction torques Tm3 and Tm4 are generated between the rotor 110 and the shaft 100. Also, friction torques Tm1 and Tm2 are similarly generated between the cylindrical portion 120 and the coil spring 130. These friction torques are added to a rotor torque T when the rotor 110 is rotated clockwise (when the rotor 110 is rotated by the deceleration), while they are subtracted from the rotor torque T, as illustrated with solid lines “B” in FIG. 4. In other words, all the friction torques Tm1, Tm2, Tm3 and Tm4 are added when the rotor 110 rotates clockwise and are subtracted when the rotor 110 rotates counter-clockwise. This means that the total torque considerably differs according to the rotational directions. FIG. 4 also shows the total torque attained in the present invention with dotted lines “A”.
Accuracy in detecting the collision impact is decreased as the difference of the total friction torques becomes large. In addition, the friction torque is not constant, but shows a considerably large dispersion. Accordingly, it is required to reduce the amount of friction torque to attain a stable and accurate performance of the detector.